In order to control a servo motor operating according to an input command, a PID control method which is a traditional control method is widely used. However, the PID control method is a linear control method, which is weak against nonlinearity of a model or occurrence of external disturbance, and the PID control method does not easily cope with the change of load in operation. For this reason, the PID control method has a drawback in that a tuning process should be performed separately for each situation. In order to overcome this drawback, various automatic tuning methods have been proposed, but these automatic tuning methods have insufficient performance in comparison to manual tuning methods.
Meanwhile, a variable structure control method which is a kind of nonlinear control ensures robustness within a range between an upper limit and a lower limit against to external disturbance such as change in parameters or load fluctuation, and is advantageously applied to a nonlinear in an easy way. Further, since an external disturbance compensator is used to decrease a value of a robustness coefficient, the chattering problem of the variable structure controller may be reduced. If this variable structure control method is applied, due to its robustness, a tuningless nonlinear controller requiring no user input may be implemented.
However, an external disturbance compensator of a general variable structure control method estimates external disturbance based on a predetermined model. For this reason, if an actual input is changed from the predetermined model (for example, an input current is limited), suitable external disturbance estimation may be failed, which greatly deteriorates the overall control performance.